Telephone or like system



July 19, 1945. ETAL 2,380,232

TELEPHONE OR LIKE SYSTEM Filed Dec. is. 1943 VFR fi? r-- '1 LV, l 14 I 1 19 E l I l 15 J INVENTORS CHARLES GILLINGS. I PERCY EDWARD ALEXANDER COWLEY ATTORNEY Patented July 10, I945 2,380,232 TELEPHONE on LIKE. SYSTEM Charles Gillings, Liverpool, and Percy Edward Alexander Cowley, Coventry, England, assignors to Automatic ElectricLaboratories, Inc., Chicago, 111., a corporation of Delaware Application December 18, 1943, Serial No. 514,810 I In Great Britain January 14, 1943 i l Claims. (01.179-84) The present invention relates to telephone or like systems and is more particularly concerned with systems in which the setting" and control of automatic switching apparatus is effected by means of voice frequency currents transmitted over the speaking leads.

It is already known that in systems of this type, particularly where connections maybe set up over considerable distances, it is generally desirable to take special steps to prevent the voice frequency signal currents from passing beyond 'the point at which their operation is to be effective in order to obviate the danger of their producing additiona1 effects at more remote parts of the system. This may be done by the use of a blocking device which may involve actuall breaking the leads over which speech currents will eventually flow or by connecting across them an effective shortcircuit.

In systems of this type conditions sometimes arise in which signals from both directions are required to be received at a particular point. In these circumstances if the receiver responsive to voice frequency signals from one direction produces a blocking effect beyond the point of connection of the receiver to the line, it will clearly be unsatisfactory for this receiver to respond to signals in the reverse direction since these would have the effect of causing the receiver to disable its operating circuit.

An obvious method of overcoming this difficulty is to provide a separate voice frequency receiver on each side of what may be termed the splitting point so that a suitable response will be obtained to signals transmitted in either direction. Such an arrangement involves additional cost and complication however, and it is the chief object of the invention to enable a single receiver to operate satisfactorily with signals from either direction while the desired blocking effect is still obtained so as to prevent the signals extending into portions of the line where their presence is not desired.

According to the invention, in a telephone system arranged for supervisory signalling over an established connection to be effected in both directions by currents within the voice frequency range each adapted to operate a voice frequency receiver which then produces a blocking effect to prevent the signal being effective at a more distant point, the use of a single voice frequency receiver responsive to signals in either direction is rendered possible by connecting it to the line on both sides of the point where the blocking effect is produced by equipment giving unidirectional transmission. j In one formof the invention the desired result is brought about by connecting the voice frequency receiver to the line on both sides of the splitting point by way of a hybrid coil while in another form the voice frequency receiver'is inductively coupled'to the line on both sides ofthe splitting point but by a thermionic valve circuit arrangement such that noappreciable transfer of Signal energy takes place across the splitting point. grammatically in Figs. 1 and 2 respectively of the accompanying drawing and both arrangements illustrate a voice frequency receiver connected up at a splitting point operating on the principle disclosed in the application of Gillings et al., filed September 18, 1943, Serial No. 502,884, whereby a virtual short-circuit is placed across the line to produce the blocking effect.

In each of the two forms of the invention illustrated in the drawing a transmission line, a resistance network, and contacts of a relay which it is intended shall be controlled by signals transmitted over the line are shown. The relay contacts are arranged to connect the resistance network in the transmission line so as to at times effectively split it into two sections. Two methods are shown for transmitting signals received over either line section to a single voice frequency receiver without interfering with the other line section. Inthe first method, shown in Fig. 1, a hybrid transformer is provided having its line coils connected between the two sections of the line on opposite sides of the resistance network and having its secondary coils connected to the input of the voice frequency receiver. In the second method, shown in Fig. 2, two transformers are employed having their primary windings connected respectively to the two line sections and having their secondary windings serially connected to the input of the voice frequency receiver.

Referring now particularly to Fig. 1, l0 and I I represent the incoming speaking leads, and I2 and 13 the outgoing speaking leads. Signals from both directions may be transmitted to the voice frequency receiver VFR over leads l4 and I5 by way of the hybrid transformer HT. The line coils of this transformer are connected by way of the resistances YE and YF which are designed.

These two arrangements are shown dia 2 i to flow over the various contacts in the speaking leads and thus obviate trouble due to so-called dry contacts. The function of the other resistances and of the relay contacts will be readily appreciated from the specification previously referred to. Briefly this is that when voice frequency currents representing numerical digits are being received, relay CA is operated and at its contacts cal, c112v and ca3 produces a virtual short-circuit across the speaking conductors while including resistances YA, YB, Y0 and YD to provide a suitable termination. Subsequently when speaking conditions are established, relay IR is operated and the termination formed. b

resistance YE is cut out and a. straightsthroughconnection is provided.

'In the arrangement of 2, the voice frequency receiver VFR is connected to theline on I each side of the splitting point by Way of a high impedance voltage circuit which includes two transformers TI and T2 the secondaries of which are in series in the grid-cathode path of the limiter valve LV of the receiver VFR. With this arrangement, voice frequency signals arriving from either direction are operative on the receiver VFR but since there is no current flow in the grid circuit, neither transformer will affect the other and the blocking effect is satisfactorily maintained.

. According to the invention therefore it is possible in a very simple manner to make one voice frequency receiver serve for signals in both directions while at the same time the requirement that the signals shall not extend further in either direction is effectively fulfilled.

What we claim as new and desire to secure by Letters Patent is:

1. In a telephone system, a bi-directional transmission line, means inserted in said line for at times preventing the transmission of signals therethrou'gh, a signal receiving circuit, and means coupling said signal receiving circuit to :said line at points on either side of said first means, whereby said signal receiving circuit is rendered responsive to signals received over the line from eitherdirection.

2."A telephone system as claimed in claim 1 in which said coupling means comprises a hybrid transformer having its line windings connected to the transmission line at said points and having its secondary windings connected to said signal CHARLES GILLINGS. PERCY EDWARD ALEXANDER COWLEY. 

